The present invention relates to a radio communication technology, and more particularly to a technology for reducing interference among base stations in a cellular radio communication system.
In a cellular radio communication system, multiple base stations or antennas are geographically dispersed and a radio communication service area is formed in a range in which the radio waves from the base stations and the antennas are received. A mobile terminal, which is capable of receiving signals from multiple base stations and antennas, is controlled so that the mobile terminal is connected to the base station from which the highest-quality signal is received. A radio communication system provides a mechanism, called a handover, via which a terminal switches the base station, to which it is connected, from one base station to another as the terminal moves, allowing the terminal to maintain the radio communication even when it is moving. To assure the connectivity of a terminal while it is moving, the boundaries of the service areas formed by the base stations and antennas overlap. The signal transmitted from each base station or each antenna, though significant information to a terminal connected to the base station and the antenna, is interference to the communication of a terminal connected to another base station or antenna. Such interference becomes an obstruction to an interfered terminal and results in a reduction in communication quality or throughput.
One known method for reducing interference among base stations is Fractional Frequency Reuse (FFR). This technology provides a mechanism in which multiple base stations and antennas put weight on the transmission power of frequency resources or select frequency resources to allow the usable frequency resources to be shared to prevent the generation of interference.
The technology FFR is based on the premise that mobile terminals are uniformly distributed. Actually, however, the distribution of mobile terminals depends on the factors such as commercial activities with the tendency that the density is high in a specific area. For example, the population density is very high at a major terminal station, but not in the area surrounding the major terminal station. As a result, there is a large difference between the number of terminals in communication with the base station covering a major terminal station and the number of terminals in communication with the base station covering an area surrounding the terminal station. Because of this, FFR that is based on the premise that the distribution of mobile terminals is uniform sometimes does not improve interference.
To achieve fairness for each mobile terminal when the distribution of mobile terminals is very uneven, it is desirable that the base stations surrounding a major terminal station limit the use of a part of frequency resources and that the reduction in interference in the limited frequency be declared to the surrounding base stations. In a part of radio communication systems using a new standard (for example, 3GPP TS36.423 V8.9.0 8.3.1 (Load Indication), hereinafter called Document 1), the interface for transmitting and receiving the information on the interference control among base stations is installed. This mechanism is called Inter Cell Interference Coordination (ICIC). In a radio communication system in which ICIC is installed, the resource state and the interference state must be reported among the base stations. FFR in which ICIC is installed is called Dynamic FFR (DFFR).